Genetic engineering of plants, which entails the isolation and manipulation of genetic material (usually in the form of DNA or RNA) and the subsequent introduction of that genetic material into a plant or plant cells, offers considerable promise to modern agriculture and plant breeding. Increased crop food values, higher yields, feed value, reduced production costs, pest resistance, stress tolerance, drought resistance, the production of pharmaceuticals, chemicals and biological molecules as well as other beneficial traits are all potentially achievable through genetic engineering techniques. Once a gene has been identified, cloned, and engineered, it is still necessary to introduce it into a plant of interest in such a manner that the resulting plant is both fertile and capable of passing the gene on to its progeny.
A variety of methods have been developed and are currently available for the transformation of various plants and plant cells with DNA. Generally, these plants have been dicotyledonous, and some success has been reported with certain of the monocotyledonous cereals. However, some species have heretofore proven untransformable by any method. Thus, previous to this discovery, no technology had been developed which would permit the production of stably transformed Zea mays plants in which the transforming DNA is heritable thereof. This failure in the art is well documented in the literature and has been discussed in a number of recent reviews (Potrykus, 1989; Weising et al., 1988; Cocking et al., 1987).
European Patent Publications 270,356 (McCabe et al.) and 275,069 (Arntzen et al.) describe the introduction of DNA into maize pollen followed by pollination of maize ears and formation of seeds. The plants germinated from these seeds are alleged to contain the introduced DNA, but there is no suggestion that the introduced DNA was heritable, as has been accomplished in the present invention. Only if the DNA introduced into the corn is heritable can the corn be used in breeding programs as required for successful commercialization of transgenic corn.
Graves et al. (1986) claim Agrobacterium-mediated transformation of Zea mays seedlings. The evidence was based upon assays known to be unreliable.
Despite extensive efforts to produce fertile transformed corn plants which transmit the transforming DNA to progeny, there have been no reported successes. Many previous failures have been based upon gene transfer to maize protoplasts, oftentimes derived from callus, liquid suspension culture cells, or other maize cells using a variety of transformation techniques. Although several of the techniques have resulted in successful transformation of corn cells, the resulting cells either could not be regenerated into corn plants or the corn plants produced were sterile (Rhodes et al. 1988). Thus, while maize protoplasts and some other cells have previously been transformed, the resulting transformants could not be regenerated into fertile transgenic plants.
On the other hand, it has been known that at least certain corn callus can be regenerated to form mature plants in a rather straightforward fashion and that the resulting plants are often fertile. However, no stable transformation of maize callus was ever achieved, i.e., there were no techniques developed which would permit a successful stable transformation of a regenerable callus. An example of a maize callus transformation technique which has been tried is the use of Agrobacterium-mediated transfer.
The art was thus faced with a dilemma. While it was known that corn protoplast and suspension culture cells could be transformed, no techniques were available which would regenerate the transformed protoplast into a fertile plant. While it was known that corn callus could be regenerated into a fertile plant, there were no techniques known which could transform the callus, particularly while not destroying the ability of the callus both to regenerate and to form fertile plants.
Recently, a new transformation technique has been created based upon the bombardment of intact cells and tissues with DNA-coated microprojectiles. The technique, disclosed in Sanford et al. (1987) as well as in EPO Patent Publication 331,855 of J. C. Sanford et al. based upon U.S. Ser. No. 07/161,807, filed Feb. 29, 1988, has been shown effective at producing transient gene expression in some plant cells and tissues including those from onion, maize (Klein et al. 1988a), tobacco, rice, wheat, and soybean, and stable expression has been obtained in tobacco and soybeans. In fact, stable expression has been obtained by bombardment of suspension cultures of Zea mays Black Mexican Sweet (Klein et al. 1989) which cultures are, however, non-regenerable suspension culture cells, not the callus culture cells used in the process of the present invention.
No protocols have been published describing the introduction of DNA by a bombardment technique into cultures of regenerable maize cells of any type. No stable expression of a gene has been reported by means of bombardment of corn callus followed by regeneration of fertile plants and no regenerable fertile corn has resulted from DNA-coated microprojectile bombardment of the suspension cultures. Thus, the art has failed to produce fertile transformed corn plants heretofore.
A further stumbling block to the successful production of fertile transgenic maize plants has been in selecting those few transformants in such a manner that neither the regeneration capacity nor the fertility of the regenerated transformant are destroyed. Due to the generally low level of transformants produced by a transformation technique, the need for selection of the transformants is self-evident. However, selection generally entails the use of some toxic agent, e.g., herbicide or antibiotic, which may be detrimental to either the regenerability or the resultant plant fertility.
It is thus an object of the present invention to produce fertile, stably transgenic, Zea mays plants and seeds which transmit the introduced gene to progeny. It is a further object to produce such stably transgenic plants and seeds by a particle bombardment and a selection process which results in a high level of viability for at least a few transformed cells. It is a further object to produce fertile stably transgenic plants of other graminaceous cereals besides maize.